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# Is it a new substance?

Year 8 Chemistry · Chemical change

<!-- Speaker note: Students state the new substance test for chemical change, explain it as atoms rearranging into new particles, and use it in place of the reversibility rule (AC9S8U07, node 2, a hinge node). About 25 minutes (Engage and Explore 8 min · Explain 10 min · Evaluate 3 min · Typical adds 4 min, Wider about 5 min). -->

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## Today

**We are learning to** a chemical change makes a new substance, because atoms rearrange into new particles. A physical change only rearranges or spreads out the same particles. "Can it be reversed?" is not the test.

- I can state that a chemical change makes a new substance.
- I can classify changes as physical or chemical with the new substance test.
- I can explain why "can it be reversed?" is not a reliable test, with a counterexample.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea; it does not replace practical work. Set the width with `?width=minimum`, `typical` (default) or `wider`. The hinge risk is the reversibility rule from AC9S6U04. Present the new substance test as a replacement, not an addition, and name the counterexamples (dissolving; recharging a battery). Teacher: In your own words: the dissolving counterexample is the evidence that the old rule has gone. -->

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## Why it matters

> You can melt ice and freeze it again. You can't unburn toast. Is that how to tell a chemical change? Dissolved salt comes back when the water dries up…

<!-- Speaker note: In Year 6 you may have learned that changes you can reverse are physical. This lesson tests that rule, and replaces it with a better one. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**Someone says: "If a change can be reversed it is physical, and if it can't be reversed it is chemical." Is that a reliable test?**

- Yes, it always works
- No, it never works
- It works for most changes but there are exceptions both ways

<!-- Speaker note: What happens: It has exceptions both ways. Dissolving salt can be reversed and is physical, but some chemical changes can be reversed too (recharging a battery). The reliable test is whether a new substance is made. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: take a show of hands for 'yes, it always works'. Come back to them at Evaluate. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fchemistry-8%2Fchemical-change%2Fstep-2&code=widget%3A+reaction%0Ashow%3A+H2+x10%2C+Cl2+x10%0Areact%3A+H2+%2B+Cl2+-%3E+2+HCl%0Aea%3A+2%0Atarget%3A+HCl+10%0Acaption%3A+%22Hydrogen+and+chlorine.+A+yellow+ring+marks+each+reaction.%22%0A}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-8/chemical-change/?full=step-2#step-2)

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: on the reaction board, point to the atom tally: same atoms before and now. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## What happened?

* It has exceptions both ways. Dissolving salt can be reversed and is physical, but some chemical changes can be reversed too (recharging a battery). The reliable test is whether a new substance is made.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## The reaction

**On the reaction board, what happened to the atoms?**

- They rearranged into new particles: hydrogen chloride
- New atoms were made
- Some atoms were destroyed
- They melted together

* Answer: They rearranged into new particles: hydrogen chloride

<!-- Speaker note: The atoms are the same; they are arranged into different particles. A new particle means a new substance. -->

<!-- Speaker note: Teacher: the sort's 'why' notes appear once each item is right; ask students to say the test aloud for two of them. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## The change of state

**When the ice melted and boiled, what happened to the water particles?**

- They stayed water particles; only their spacing and motion changed
- They became steam particles, a new substance
- They turned into hydrogen and oxygen

* Answer: They stayed water particles; only their spacing and motion changed

<!-- Speaker note: A change of state is physical: the same particles, arranged and moving differently. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Physical or chemical

**Physical or chemical? Use the new substance test.**

Groups: Physical · Chemical

Ice melting · Salt dissolving in water · Paper burning · An iron nail rusting · Cooking an egg · Water boiling · Baking soda fizzing in vinegar · Chocolate melting

* Answer: Ice melting → Physical; Salt dissolving in water → Physical; Paper burning → Chemical; An iron nail rusting → Chemical; Cooking an egg → Chemical; Water boiling → Physical; Baking soda fizzing in vinegar → Chemical; Chocolate melting → Physical

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## When the old rule fails

**Dissolving salt can be reversed, and it is physical. Which of these breaks the old "can it be reversed?" rule the other way?**

- Cutting paper
- Freezing water
- Recharging a battery: a chemical change that can be reversed

* Answer: Recharging a battery: a chemical change that can be reversed

<!-- Speaker note: Charging and using a battery runs chemical reactions forwards and backwards. Reversibility is a clue, not the test. -->

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## Elaborate: is dissolving chemical?

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fchemistry-8%2Fchemical-change%2Fstep-4&code=widget%3A+dissolve%0A}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-8/chemical-change/?full=step-4#step-4)

<!-- Speaker note: What this step is for: Dissolving is the classic ambiguous case: the solid seems to disappear, but the particles are unchanged. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Disappear, or spread out?

**The salt disappeared. Is dissolving a chemical change?**

- No: the salt particles are still there, spread between the water particles
- Yes: the salt turned into water
- Yes: the salt was destroyed

* Answer: No: the salt particles are still there, spread between the water particles

<!-- Speaker note: No new substance: physical. The salt particles spread out between the water particles. -->

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## Elaborate: burning

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-science/index.html?id=deck%2Fscience%2Fchemistry-8%2Fchemical-change%2Fstep-5&code=widget%3A+reaction%0Ashow%3A+CH4+x6%2C+O2+x12%0Areact%3A+CH4+%2B+2+O2+-%3E+CO2+%2B+2+H2O%0Aea%3A+1.5%0Arelease%3A+1%0Acaption%3A+Methane+%28natural+gas%29+burning+in+oxygen.%0A}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-8/chemical-change/?full=step-5#step-5)

<!-- Speaker note: What this step is for: Burning is the change students most often describe as 'destroying' the fuel; watching the atoms go into new products counters that. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Where did the atoms go?

**When methane burns, what happens to its atoms?**

Groups: True · False

The carbon atoms end up in carbon dioxide · The hydrogen atoms end up in water · The methane is destroyed and turns into energy · The gas gets hotter

* Answer: The carbon atoms end up in carbon dioxide → True; The hydrogen atoms end up in water → True; The methane is destroyed and turns into energy → False; The gas gets hotter → True

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## Evaluate: look back

Write one sentence: "At first I thought… Now I think… because…"

**Revision summary**

- A **chemical change** makes a **new substance**: atoms separate and join into new particles.
- A **physical change** (melting, boiling, dissolving) keeps the same particles.
- **"Is there a new substance?"** is the test. "Can it be reversed?" is only a clue: dissolving can be reversed and is physical.
- In a chemical change no atoms are made or destroyed; they are rearranged.

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Exit ticket

**Is rusting physical or chemical? How do you know?**

* Chemical: rust (iron oxide) is a new substance with different properties from iron.

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this leads: Next in the unit: indicators of chemical change (node 3) and energy in reactions (nodes 4 and 5). In class: steel wool in vinegar (rust over a few days), and dissolving then evaporating salt water, side by side. -->